1. Field of the Invention
This invention relates to an apparatus for the production of graphite fibers from preoxidized fibers or carbon fibers and more particularly to a heating apparatus which permits the production of graphite fibers having stable quality from preoxidized fibers or carbon fibers continuously for a long period of time.
2. Description of the Prior Art
As is known in the art, a high frequency induction heating apparatus can be used in producing graphite fibers by heating preoxidized fibers or carbon fibers, particularly pre-oxidized fibers or carbon fibers derived from polyacrylonitrile fibers, in an inert gas atmosphere at a temperature of 2000.degree. C. or more.
In such a high frequency induction heating apparatus, high temperatures of 2,000.degree. C. or more, particularly 2,500.degree. C. or more can easily be produced in a short time by use of an oscillator for high frequency induction heating, and the temperature of a heating element can be stabilized by controlling the variation of high frequency output to .+-.0.1% relative to a .+-.10% variation of the power voltage, by using an electric control circuit.
However, a change in the quantity of emitted heat in such a heating apparatus, particularly an increase in the quantity of emitted heat which results from deterioration of an insulating material due to high temperatures, cannot be prevented. This leads to instability in the temperature in the heating apparatus, and often to a decrease or to variations in the quality of the graphite fibers formed.
As heat-insulating materials for use in a high temperature furnace in which the temperature is 1,000.degree. C. to 3,000.degree. C., carbon fiber felts, graphite powders, powdery carbon black, etc. have hitherto been employed, including graphite powders having average diameters in the range of from 20 to 50.mu. and powdery carbon black having average diameters in the range of 20 to 500 m.mu. are known.
The "average diameter" of a grain is measured using a photomicroscope or a scanning electron microscope. The average of the shortest diameter and longest diameter of each grain is obtained and is defined as the diameter of a grain. In such a manner, the diameters of 100 particles are measured and the average diameter is obtained therefrom.
These conventional heat-insulating materials, however, when used in a heating apparatus of the high frequency induction system type, suffer from disadvantages as described below.
Where a heat-insulating material of graphite powder is used in the high frequency induction type heating apparatus, it is easily induced by the high frequency oscillation, and acts not only as a heat-insulating material, but also as part of a heating element. Therefore, its effect as a heat-insulating material is reduced, and it is necessary to increase the thickness of the heat-insulating material layer. Moreover, at high temperatures of 2,000.degree. C. or more, the graphite powder is subject to deterioration in quality, reduction in electric resistance due to an increase in the degree of crystallinity, etc., and the quantity of heat generated by high frequency changes with a lapse of time, resulting in a variation in the temperature of the heating apparatus.
On the other hand, powdery carbon black is not induced by a magnetic field which is produced by high frequency oscillations, as is the case with the heat-insulating material of graphite powder, because it has a very fine particle having a average diameter in the range of from 20 to 500 m.mu., and it therefore exhibits a capability to act as a heat-insulating material. However, at high temperatures of 2,000.degree. C. or more, there occurs deterioration of the carbon, reduction in the volume of carbon (which is caused by graphitization), and the formation of voids in the carbon black layer (which is caused by the decomposition and dissipation of organic substances in the carbon black and the expansion and shrinkage of atmospheric gas). Therefore, the quantity of emitted heat increases, resulting in a drop in the temperature in the heating apparatus.
Increasing the output of the oscillator to compensate for the increase in the quantity of emitted heat (that is, in order to raise the temperature) is subjected to limitations and, moreover, gives rise to the problem that the temperature of the outer wall (comprising a glass material) of the heating apparatus may exceed the maximum that it is capable of withstanding.
Therefore, periodic recharging of the heat-insulating material has been required in order to prevent such an increase in the quantity of emitted heat, making it difficult to maintain the operation of such apparatus continuously for a long period of time. Furthermore, since the temperature in the heating apparatus are very high, it is necessary to use as an atmospheric gas, an inert gas, particularly argon, which is very expensive, and it has therefore been desired to employ an apparatus structure which permits a reduction in the amount of the argon used to as low a level as possible.